Electronic components are often mounted on a printed wiring board (also known as a “printed circuit board”) via a process known as “surface-mount technology” (SMT). In SMT, the electrical components are soldered to pads on the surface of the printed wiring board instead of in plated holes drilled through the board, as in through-hole technology.
In a known technique for printing solder paste on a printed wiring board, and as shown in FIG. 1, a blade 12 (e.g., of a squeegee) pushes solder paste 14 along a fill side 16 of a stencil 18 and then through apertures in the stencil 18 and onto a corresponding pattern of conductive contact pads 20 on a surface of the printed wiring board 22. The stencil 18 is a substantially planar, thin metal or plastic plate with apertures formed therein. The purpose of the stencil 18 is to allow solder paste, adhesives or other materials used in the electronics industries to be applied, or squeezed, through the aperture openings onto the contact pads 20 without free solder paste, adhesive or other materials being misplaced, missing one or more contact pads 20, and erroneously being applied on the surface of the printed wiring board 22.
Conventional thinking in the stencil fabrication industry has held for years that the release of paste from a stencil aperture is enhanced if the aperture walls 24, 26 are tapered so that the aperture's widest opening is on the board side 28 of the stencil 18, while its narrowest opening is on the stencil's opposite, “fill” or “squeegee” side 16. Henceforth, this configuration, illustrated in FIG. 1, is referred, to as an acute aperture taper.
In support of the case for creating the walls with an acute taper, careful measurements of the amount of material deposited during printing as a percentage of the aperture's theoretical volume shows an increase in the percentage of material that is deposited, at least to a degree, as the acuteness of the taper of the walls from the fill side to the board side of the stencil is increased.
Even more careful measurements have shown that this observed improvement in relative release efficiency is due to a simultaneous increase in the so-called area factor of the aperture, which is a consequence of the tapering process. “Area factor” is defined as the ratio of the area of the stencil aperture at the board side to the area of the aperture walls. The underlying physical theory is that transfer of paste to the pad is enhanced by having a large area of contact between the paste and pad; conversely, paste more readily adheres to the aperture walls in preference to the pads where there is a large contact area between the paste and the wall surfaces, and hence, a low area factor. However, this improvement may be offset because the taper forces the capillary force to work at an angle to the perpendicular.